Grid Outages and the Magic of Frequency Shifting

July 10, 2019

This is the fascinating story of frequency shifting.

Normal electric utility frequency is 60 hertz (Hz). Solar electric inverters require the utility frequency to be at or near 60 Hz in order to operate. During a grid outage, Powerwall effectively establishes grid quality power (120/240 volts at 60 Hz), allowing a nano grid to operate at your home with solar providing power to your loads (during daylight hours) and charge Powerwall as needed.

Under grid outage conditions, during the day with the house loads being met by your PV system and Powerwall being at or near full state of charge (97-100 percent), Powerwall will shift the frequency from 60 to 66 Hz in order to effectively turn off the PV inverter(s). That is, the PV inverter(s) see the frequency out of the normal spec and shut down as they’re expected and required to do. Why? Because with Powerwall at or near full and the house loads being met, there’s nowhere for any additional solar generation to go. Powerwall will wait for its the state of charge to drop below 96-97 percent before shifting the frequency back to 60 Hz, which, with a five-minute delay, allows the PV system to restart and generate solar power again. This frequency shifting can take place repeatedly over the course of the day depending on load demands, solar potential and Powerwall state of charge and is perfectly normal and does not damage the PV inverters.

So if you happen to be looking at the Tesla app during a grid outage during the day and see that there’s no solar power being generated, that’s because of the frequency shifting phenomenon and does not indicate any kind of equipment failure.

Also worth noting: when Powerwall is at 66 Hz, some of your house loads may be affected. What we and other Powerwall owners have noticed, while not a comprehensive list, can include: electronic clocks running fast, motors sounding different (microwave, washing machine, pumps), uninterruptible power sources (UPS) not charging and going into back-up power mode, appliances having a computer behaving unusually.